Flow batteries are electrochemical energy storage systems in which electrochemical reactants are dissolved in liquid electrolytes (sometimes referred to generically as “reactants”), which are pumped through reaction cells where electrical energy is either converted to or extracted from chemical potential energy in the reactants by way of reduction and oxidation reactions. In applications where megawatts of electrical energy must be stored and discharged, a redox flow battery system may be expanded to the required energy storage capacity by increasing tank sizes and expanded to produce the required output power by increasing the number or size of electrochemical cells or cell blocks. A variety of flow battery chemistries and arrangements are known in the art.
In some redox flow battery systems based on the Fe/Cr redox couple, the catholyte (in the positive half-cell) contains FeCl3, FeCl2 and HCl. The anolyte (in the negative half-cell) contains CrCl3, CrCl2 and HCl. Such a system is known as an “unmixed reactant” system. In a “mixed reactant” system, the anolyte also contains FeCl2 and the catholyte also contains CrCl3. In an initial state of either case, the catholyte and anolyte typically have equimolar reactant concentrations.
After a number of charge/discharge cycles, the catholyte and anolyte may become imbalanced because of side reactions during a charge and/or discharge operations. For example, in the case of an Fe/Cr redox flow battery, a hydrogen generation side-reaction occurs at the anode during the charge cycle. Such side reactions cause an imbalance in electrolyte concentrations by converting more reactant in one half-cell to a higher SOC state than occurs in the second electrolyte. In this unbalanced state, for example, the concentration of Fe3+ may be higher than that of Cr2+. The imbalance decreases capacity of the battery and is undesirable. The proportion of hydrogen gas generated, and thus the degree of reactant imbalance, also increases as the state-of-charge (SOC) of the flow battery increases.
The imbalanced state may be corrected by processing the catholyte in a rebalancing cell. One example is an Iron/Hydrogen fuel cell as described in U.S. Pat. No. 4,159,366, which describes an electrolytic rebalance cell configured to oxidize waste hydrogen at a rebalance cell anode and reduce excess Fe3+ ions to Fe2+ ions at a rebalance cell cathode. H2 may be recycled from the Cr species electrolyte and directed into the rebalance cell along with a portion of the Fe electrolyte. A catalyst may be used to promote the reaction with or without application of an applied voltage. Another example of a similar cell is provided in “Advancements in the Direct Hydrogen Redox Fuel Cell” by Khalid Fatih, David P. Wilkinson, Franz Moraw, Alan Ilicic and Francois Girard, published electronically by the Electrochemical Society Nov. 26, 2007.
Monitoring or measuring the state of charge and the imbalance of electrolytes presents additional challenges. Such concentrations may be measured spectroscopically, as described for example in U.S. Pat. No. 7,855,005 to Sahu, or by any number of other methods.